I think we all can agree that when it comes to increasing useable horsepower, four areas that can help achieve this are increasing the engine's mechanical efficiency(ME), thermal efficiency(TE), volumetric efficiency(VE) and combustion efficiency(CE). Let's take a look.
Engineers refer to powertrain drag as “parasitic drag”. It seems to fit. It’s almost like a parasite leaching power away from any component with closely fitted parts. This "parasitic drag" decreases the mechanical efficiency(ME) of an engine through frictional, inertial and pumping losses.
The (ME) is a ratio of the "power produced" to the "power doing work". Engine parts, especially main and rod bearings, pistons and valve train components, are subject to high frictional losses and can account for about 10% of total engine heat and up to 10% of the total power loss. The higher the engine operating rpm, the higher the frictional losses.
Engine accessories such as water pump, power-steering pump, alternator and clutches all have to overcome friction to operate and place more load on the engine. Inertial drag is closely related to frictional drag in this instance. Reducing inertial drag, (less rotating mass) which increases the (ME), can also affect frictional drag. Installing lighter pistons, a lighter flywheel or by installing underdrive accessory pulleys are ways of reducing inertial drag. Even more horsepower in an engine can be gained by reducing frictional losses and increasing it's (ME). A 20% reduction in friction pressure will yield approximately a 5% increase in efficiency.
Reducing frictional drag first involves metal separation. If metal separation can be achieved, not only can heat and friction be greatly reduced, but metal wear and metal transfer can also be greatly reduced too. This can extend component longevity as well.
Lubricant film strength is determined by its ability to separate metal under stress. The higher the stress the higher the lubricant film strength needed for metal separation. Lubricant film strength specifications are partially determined by viscosity, and partly determined by the lubricity (slipperiness) of the additive package.
Viscosity, or oil thickness, characterizes its fluidity, or its flow ability. The viscosity can be expressed as "millimeters squared per second" (mm^2/s). A higher viscosity oil increases viscous friction, or oil drag, but usually has a higher film strength. Viscosity specifications are usually determined by the oil clearances of the application. Larger oil clearances have a thicker layer of oil for maximum metal separation (hydrodynamics), but the increase in oil drag decreases (ME).
Smaller oil clearances are a growing trend, especially in smaller displacement, high rpm engines. In order to increase (ME), a significantly thinner layer of oil is needed with a higher risk of metal transfer and wear. Thin film strength becomes critical at this point, and this is where the lubricity plays more of a role in metal separation since thinner viscosity oils are often employed. Not only does the thin layer of oil have to endure more heat from the tighter clearances, it has to have high thin film strength and be able to cling to and work into the pores of the metal for complete metal separation and less oil drag. This means more(ME) and more usable horsepower!
Frictional drag and oil drag can also be reduced in the drivetrain for optimum efficiency. Frictional losses in the drivetrain can be as much as 15% of the total power loss, depending on the application. Manual gearboxes, transfer cases, rear gears/ differentials and power steering pumps are all areas where an increase in efficiency can mean more power.
The second way we can increase horsepower is by increasing cylinder pressure. This means increasing cylinder combustion temperatures without going too "hot" and causing pre-ignition or detonation. This is usually achieved three separate ways, all essentially producing the same results, without having to increase bore/stroke. They are:
2. Increasing compression ratio or thermal efficiency(TE)
3. Optimizing the ignition timing points and spark intensity
Increasing the thermal efficiency(TE) of an engine means more heat can be converted to mechanical energy at the piston. The (TE) is a ratio of "heat produced" to "heat that produces work". Typically less than 33% of the combustion heat is producing work. The heat losses are mostly due to heat absorbtion from the cooling system and from the exhaust gases. Increasing the compression ratio(CR) is a common way to increase (TE). Special metal coatings are sometimes employed to reflect heat, rather than absorb it to help minimize total heat losses.
Optimizing the ignition timing points and increasing the spark kernel intensity can increase peak cylinder pressures and can increase the combustion flame speed, or the burn rate of the fuel. This can help increase combustion efficiency(CE), which is a ratio of "total fuel burnt" to "total fuel that was available to burn". If more fuel could be burned, then more heat can be produced to perform work on the piston. More of this subject is covered in another tech page on racing fuels.
We will be looking mostly at option one volumetric efficiency(VE), which is a ratio of "total air-fuel mass in a cylinder" to "total air-fuel mass that a cylinder can hold". Increasing (VE) means charging a cylinder with a denser air-fuel(AF) mass. In theory an engine is operating at 100% [(VE)=100%] at wide open throttle operation, when the intake manifold is fully pressurized.
At part throttle [(VE)<100%] the intake manifold has more vacuum, which reduces the (AF) mass charging the cylinders. The more vacuum present in the manifold, the more cylinder pumping and (AF) mass losses. Pumping losses decrease the overall (ME) of an engine because the pistons work harder to draw in an (AF) mass.
Whenever [(VE)>100%] , the (TE) will increase because the (AF) mass is denser and will increase compression pressures and temperatures. This will increase (CE) as the higher pressures and temperatures will increase flame speed and completeness of burn.
Because air is made up of only 21% oxygen, even in an unrestricted naturally aspirated engine, it is difficult to achieve 100% volumetric efficiency during wide open throttle cylinder charging with only 14.7 psi (atmospheric air pressure) behind it. Three ways to increase (VE) are:
1. Forced induction (supercharger / turbocharger)
2. Nitrous oxide injection
3. Chemical oxygenating agents
Superchargers and turbochargers are probably the most popular ways to increase cylinder pressure and (VE). Air pressure can be increased or boosted several psi to increase oxygen content during cylinder charging. These systems are usually very expensive, require engine compartment reconfiguration and specific ignition timing and fuel ratio tuning maps. Intake charge cooling (charge air cooling) is also usually required as the compressed air is hotter.
Nitrous oxide injection is another popular way to increase cylinder pressure and (VE) by injecting nitrous oxide and extra fuel into the air induction system. This can charge a cylinder to well over 100% efficiency depending on amount injected. It also provides thermal-charging by cooling the incoming air. These systems are moderately priced, requires little engine compartment reconfiguration, allows adjustable cylinder charge rates and (if used in small enough amounts) usually requires no tuning. Nitrous bottles have to be refilled though.
Chemical oxygenating agents or "chemical supechargers" is another way to increase cylinder pressure and (VE) by releasing oxygen in the cylinders during combustion through the fuel induction system. This can also charge a cylinder to well over 100% efficiency depending on what ratio the product is mixed with fuel. This also increases flame speed and (CE) because of the oxygen content in the fuel, reducing the need for as much (AF) mixing. There is no system to install and requires no engine compartment reconfiguration. It allows you to adjust cylinder charge rates and can be used in small enough amounts to require no tuning. The product needs to be replaced like fuel.
Restoring lost horsepower:
Fuel and oil deposits in an engine (mostly on street operated vehicles) affect power and performance mostly in three ways:
1. Decreases A/F mass during cylinder charging (VE)
2. Reduces cylinder compression pressures (TE)
3. Reduces fuel efficiency (CE)
Carbon fuel deposits on intake valves can restrict measurable air and fuel flow (cfm) per cylinder depending on the amount of build up. They can also affect proper valve closing resulting in compression loss. Deposits on piston tops can interfere with piston cooling and cause detonation or pre-ignition. Varnish/sludge/carbon deposits on oil control rings or compression rings can result in additional cylinder compression loss and cylinder wall/ring wear. Regular cleaning in these two areas are recommended for proper air/fuel ratio and maximum cylinder compression.